Apparatus for stabilizing vertebral bodies

ABSTRACT

A dynamic stabilization apparatus comprises elongated members mounted within the proximal end of anchoring devices that are placed in adjacent vertebral bodies. A flexible element having elastic properties within the applicable range of loading, for example loads that the spine experiences, is disposed between the proximal ends of the elongated members. At least one additional flexible element is mounted about the proximal ends of the elongated members adjacent the central flexible element. A housing encapsulates the proximal ends of the members such that the flexible element and the additional flexible elements are contained therein. As compressive, tensile, angular, shear and rotational forces are applied to the elongated members the central flexible element and the additional flexible elements interact with the elongated members and the housing to allow for motion of the elongated members.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. application Ser. No.12/190,423, which was filed on Aug. 12, 2008 and which is incorporatedherein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to stabilization of the vertebrae of thespinal column and, more particularly, to an apparatus whereby securingmembers are implanted and fixed into a portion of a patient's spinalcolumn and a longitudinal member including flexible, semi-rigid rod-likestructures of various cross-sections (hereinafter referred to as “rods”)are connected and fixed to the upper ends of the securing members toprovide stabilization of the spinal column.

BACKGROUND

Degenerative spinal column diseases, for example, disc degenerativediseases (DDD), spinal stenosis, and spondylolisthesis can be correctedby surgical procedures. Typically, spinal decompression is the firstsurgical procedure that is performed and results in the reduction ofpressure in the spinal canal and on nerve roots located therein. Spinaldecompression seeks to remove tissue that is applying pressure to thenerve bundle and thus relieve pain. This can result, however, inweakening the spinal column.

Certain surgical procedures, for example posterolateral fusion wherebyadjacent vertebral bodies are fused together is often necessary torestore spinal stability following the decompression procedure. Fusionof adjacent vertebral bodies requires that the bone grow together andemploys a bone graft or other biological growth agent. In order tomaintain the grafting material in place and preserve stability duringbone growth, a spinal fixation device is typically used to support thespinal column until a desired level of fusion is achieved. Depending ona patient's particular circumstances and condition, a spinal fixationsurgery can sometimes be performed immediately following decompression,without performing the fusion procedure. The fixation surgery isperformed in most cases because it provides immediate postoperativestability and, if fusion surgery has also been performed, it providessupport of the spine until sufficient fusion and stability has beenachieved.

Conventional methods of spinal fixation utilize a rigid spinal fixationdevice to support and prevent movement of an injured spinal part. Theseconventional spinal fixation devices include: fixing screws configuredto be inserted into the spinal pedicle or sacrum to a predetermineddepth and angle, rods or plates configured to be positioned adjacent tothe injured spinal part, and coupling elements for connecting andcoupling the rods or plates to the fixing screws such that the injuredportion of the spin is supported and held in a relatively fixed positionby the rods or plates. The connection units prevent further pain andinjury to the patient by substantially restraining the movement of thespinal column.

Because the connection units prevent normal movement of the spinalcolumn, after prolonged use, the spinal fixation device can cause illeffects, such as adjacent level syndrome (transitional syndrome) orfusion disease that result in further complications and abnormalitiesassociated with the spinal column. The high rigidity of the rods orplates used in conventional fixation devices causes these disorders dueto the patient's joints being fixated by the nature of surgery. Themovement of the spinal joints located above or under the operated areais increased. Consequently, such spinal fixation devices cause decreasedmobility of the patient and increased stress and instability to thespinal column joints adjacent to the operated area.

It has been reported that excessive rigid spinal fixation is not helpfulto the fusion process due to load shielding. As an alternative,semi-rigid spinal fixation devices have been utilized to address thisproblem while assisting the bone fusion process. For example, U.S. Pat.No. 5,375,823—Navas and U.S. Pat. No. 6,241,730—Alby each disclose apiston configuration mounted between fixing screws having a flexiblematerial or spring element enclosed within a sleeve allowing for axialdampening Although providing for a greater range of motion than a fixedrod, these devices fail to accommodate for a full range of physiologicalmotion, for example axial torsion or twisting, and are not well-suitedfor spinal stabilization absent fusion. Thus, in the end these devicesdo not fully prevent the problem of rigid fixation resulting fromfusion.

To solve the above-described problems associated with rigid fixation,semi-rigid and generally flexible devices have been developed. U.S.Publication No. 2006/0264940—Hartmann discloses a flexible springelement connected to a rod and an axially opposed hollow body. Thespring element and hollow body have corresponding bores that receive aclamping element. The clamping element has a convex face that abuts theend wall of the internal bore of the spring element during deformationof the spring element under axial loading of the device. The shape ofthe end of the clamping element controls the spring characteristics ofelement. While this device functions to provide a greater range ofmotion during compression it relies upon the spring element as a loadbearing structure in tension. This is not an optimal design to handlethe long-term cyclical loading the device will experience whenimplanted.

U.S. Pat. No. 5,672,175—Martin discloses a flexible spinal fixationdevice which utilizes a flexible rod made of metal alloy and/or acomposite material. Additionally, compression or extension springs arecoiled around the rod for the purpose of providing de-rotation forces onthe vertebrae in a desired direction. However, this approach isprimarily concerned with providing a spinal fixation device that permits“relative longitudinal translational sliding movement along [the]vertical axis” of the spine and has a solid construction with arelatively small diameter in order to provide a desired level offlexibility. Because they are typically very thin to provide suitableflexibility, such a rod is prone to mechanical failure and have beenknown to break after implantation in patients. Similarly, U.S.Publication No. 2007/0270814—Lim shows a vertebral stabilizer that hasmobility during compression, extension and rotation. A connecting membersuch as flexible rods, cables or braided steel are anchored at theirdistal and proximal ends to engaging portions and are coaxially locatedwithin a flexible member. While the connecting members can bend toaccommodate shear when the spine is twisted this device has been shownto fail due to fatigue once implanted.

There is no spinal fixation device that can provide for a full range ofphysiological motion when implanted in a patient. In addition, fewdevices that attempt to accommodate a range of physiological motion canwithstand long-term loading conditions. Therefore, there is a need foran improved dynamic spinal fixation device.

SUMMARY

Elongated members such as rods, plates and the like are often mounted tospan vertebral bodies in order to provide stability to localized regionsof the spine. These devices are typically mounted to the vertebralbodies via an anchoring device such as a member having threads at itsdistal end, allowing for attachment to the spine and a proximal end thataccepts the elongated member. For example, at least two threaded membersare placed in adjacent vertebral bodies and the elongated members aremounted to the proximal end of threaded members so as to span thevertebral bodies. Rigid elongated bodies are typically employed in orderto prevent motion between the vertebral bodies.

According to the invention, a dynamic stabilization apparatus isprovided. The apparatus comprises elongated members such as rods mountedwithin a housing. The elongated members are mounted within the proximalend of anchoring devices that are placed in adjacent vertebral bodies. Acentral flexible element having elastic properties within the applicablerange of loading, for example loads that the spine experiences, isdisposed between the proximal ends of the elongated members. At leastone additional flexible element is mounted about the proximal ends ofthe elongated members adjacent the central flexible element. The housingencapsulates the proximal ends of the members such that the centralflexible element and the additional flexible elements are containedtherein. As compressive, tensile, angular, shear and rotational forcesare applied to the elongated members the central flexible element andthe additional flexible elements interact with the elongated members andthe housing to allow for motion of the elongated members. The degree ofpermissible motion may be varied, for example, by varying the materialfrom which the flexible members are constructed.

The housing may be generally cylindrical and has openings at each endfor receiving the elongated members there through. In one embodiment ofthe invention, the housing is constructed from a generally rigidmaterial that will not deform under the physiological loadingencountered within the spine. The housing may be formed from a first anda second casing wherein each of the casings have an opening therein. Thecasings include an engagement feature such that after the elongatedmembers are inserted through the openings the casings are engagedtogether to assemble the apparatus.

In one embodiment of the invention the proximal ends of the elongatedelements are larger than the central and distal portions of theelongated member. For example, one or both of the proximal ends areflanges. The flange includes an inward surface facing the centralflexible element and an outward surface facing the distal end of saidelongated member. The inward surface may be generally concave andcontacts an outer facing surface of the central flexible clement that isconvex. Alternatively, the inner surface of the flange may be convexwhile the contacting or outer surface of the central flexible element isconcave. A variety of shapes for the two surfaces may be employedincluding having both surfaces be flat.

The central flexible member may be constructed from a polymer and has afirst and a second outward facing surface. The central flexible memberresists rotational and compressive forces. The inward surface of each ofthe flanges contacts the outer facing surfaces of the central flexibleelement. Protrusions located on either the outward surface of thecentral element or the inward surface of the flanges engages withcorresponding recesses to form an anti-torsional coupling. As theelongated members are rotated about their axis in opposite directionsthe engagement of the protrusions within the recesses causes the centralflexible element to elastically deform, resisting the motion. Inaddition, as the elongated members experience a compressive force theflanges engage the central flexible element. The central flexibleelement is compressed resisting while allowing motion of the elongatemembers. Eventually the central flexible element deforms such that itcontacts the housing further increasing the resistance to the motion ofthe elongated members.

The outward surface of the proximal end of the elongate members or theflange contacts a surface of the additional flexible element. A varietyof shapes can be employed for the outward surface of the flange and thecorresponding contacted surface of the additional flexible element. Theshaping of these surfaces may be varied in order to create a desireddynamic response. As with the central element, the additional flexibleelements may be constructed from a polymer. The additional flexibleelements serve as an axial and radial buffer between the housing and theelongated members. For example, as the elongated members are subjectedto an axial or radial force, the flange pushes on and deforms theadditional flexible member which resists the motion of the elongatedmembers. Varying the elastic properties of the central and flexiblemembers allows the load-displacement response of the apparatus to becustomized.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the invention will be apparent to thoseof ordinary skill in the art from the following detailed description ofwhich:

FIG. 1 is an isometric view of to an embodiment of the presentinvention;

FIG. 2 is an exploded view of the components of an embodiment of thepresent invention.

FIG. 3 is a side view of an embodiment of the present invention.

FIG. 4 is a view of an embodiment of the present invention taken alongline 4-4 of FIG. 3.

FIG. 5 is an isometric view of an embodiment of the central element ofthe present invention.

FIG. 6 is an alternative embodiment of the central element of thepresent invention.

FIG. 7 is an alternative embodiment of the central element of thepresent invention.

FIG. 8 is an alternative embodiment of the central element of thepresent invention.

FIG. 9 is an alternative embodiment of the central element of thepresent invention.

FIG. 10A is a cross section view showing an embodiment of the presentinvention in an unloaded state.

FIG. 10B is a cross section view showing an embodiment of the presentinvention under tension.

FIG. 11A is a cross section view showing an embodiment of the presentinvention in compression.

FIG. 11B is a cross section view showing an embodiment of the presentinvention in a further compressed state.

FIG. 12 is a posterior view showing an embodiment of the presentinvention placed on a section of the spine.

FIG. 13 is a side view showing an embodiment of the present inventionplaced on a section of the spine whereby the section of the spine is inflexion.

FIG. 14 is a side view showing an embodiment of the present inventionplaced on a section of the spine whereby the section of the spine is inextension.

FIG. 15 is a posterior view showing an embodiment of the presentinvention placed on a section of the spine whereby the section of thespine experiences lateral bending.

FIG. 16 is a posterior view showing an embodiment of the presentinvention placed on a section of the spine whereby the section of thespine experiences axial rotation.

FIG. 17 is a cross section view showing an embodiment of the presentinvention actuated under shear.

FIG. 18 is a cross section view showing an embodiment of the presentinvention angulated.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

An implantable dynamic apparatus for stabilizing a desired region of thespine will be described with reference to FIGS. 1-18. As shown in FIGS.1-4 the apparatus 10 of the present invention generally compriseselongated members 40 a, b mounted within a housing 20. A central element50 having elastic properties is disposed between the proximal ends ofthe two elongated members 40 a, b. At least one additional flexible orcompressible element 30 a, b is mounted about the proximal end of theelongated members 40 a, b adjacent the central element 50. The housing20 encapsulates the proximal ends of the elongated members 40 a, b suchthat the central element 50 and the at least one additional flexible orcompressible element(s) 30 a, b are contained therein.

The elongated members 40 a, b may be constructed from materials havingsufficient strength and rigidity to resist fracture and plasticdeformation under the loads experienced by the spine. Materials such astitanium, titanium alloy, stainless steel or a polymer such as PEEK orcarbon fiber may be employed. The elongated members 40 a, b may have avariety of shapes such as cylindrical or polygonal and need not bothhave the same shape. The construction of the components of the apparatus10 may be varied to meet the particular conditions of the patient inwhich the apparatus 10 will be utilized. For example, the material usedto construct the central element 50 may be varied or the size and shapeor the elongated members 40 a, b can be varied such that each member hasa different shape or is constructed from a different material.

As shown in FIG. 12, one or more of apparatus 10 can be mounted betweenadjacent vertebral bodies 8 a, b in order to provide stability tolocalized regions of the spine 2. Typically, stabilization devices suchas apparatus 10 are mounted to the vertebral bodies 8 a, b via ananchoring device 3. The device may comprise a member having threads atits distal end, not shown in the drawings that allow for attachment tothe honey tissue of the spine and a proximal end 4 that accepts thedistal ends of elongated members 40 a, b. Alternatively, the distal endmay comprise a clamp or other gripping surface. A retaining member 6locks the distal ends of the elongated members 40 a, b to the anchoringdevices 3. As will be described in greater detail below, when the spineexperiences the normal range of physiological motion the central element50 and the additional flexible or compressible elements 30 a, b interactwith the elongated members 40 a, b and the housing 20 to stabilize thespine while allowing for controlled movement of the adjacent vertebralbodies 8 a, b.

As shown in FIGS. 2 and 4, the central flexible element 50 is situatedbetween the proximal ends of the elongated members 40 a, b. The shape ofthe central element 50 is designed to maximize contact with the proximalends of the elongated members 40 a, b as described in greater detailbelow, while leaving a space 70 between element 50 and the housing 20 toallow for distortion of the shape of the central flexible element 50when elongated members 40 a, b are moved inward.

The central flexible element 50 can be homogeneous or made as acomposite to tailor its performance to the particular loading apparatus10 experiences when implanted. In one embodiment of the presentinvention, the central flexible element 50 is constructed from anincompressible elastomer such that as elongated members 40 a, b aremoved inwards, element 50 experiences transverse strain in response toaxial strain. A flexible material having a durometer range of 30-65 onthe Shore D scale or 20-95 on the Shore A scale and an elongation atbreak in the range of 200-600% per ASTM D-638 may be utilized toconstruct the central flexible element 50. The material utilizedpreferably is biocompatible and exhibits a consistent dynamic responseand resists wear over the millions of loading cycles experience by theapparatus 10 when implanted in the spine. One such material isPolycarbonated Polyurethane or PCU known commercially as Chronoflex. Onegrade of Chronoflex that has been shown to function with the presentinvention is Chronoflex C 55D.

As shown in FIG. 10A the central element is unexpanded when theapparatus 10 is in a neutral, unloaded position. As shown in FIGS. 11Aand B, when members 40 a, b are moved in the direction of arrows 62,such as would be experience when the spine is extended, element 50eventually expands into space 70 contacting the inner wall of housing20. With further movement of the members 40 a, b, the element 50 furtherexpands into interstitial space 28. The expansion of the element 50 intospaces 70 and under certain conditions space 28 causes an exponentialincrease in resistance to compressive loading. This allows for therestricted movement of the adjacent vertebral bodies that apparatus 10spans while also providing stability thereto.

As shown in FIG. 2, the proximal ends of the elongated arms or members40 a, b are larger than the central and distal portions of the elongatedmembers 40 a, b. For example, one or both of the proximal ends compriseflanges 42 a, b. The flanges 42 a, b include outer surfaces 46 a, bfacing the distal end of the elongated members 40 a, b and innersurfaces 48 a, b that face the central element 50. As shown in FIG. 4,the inner surfaces 48 a, b of the flanges 42 a, b may be convex whilethe opposing surfaces 54 a, b of the central element 50 are concave.Alternatively, the inward surfaces 48 a, b may be generally concavewhile the opposing surfaces 54 a, b are convex. In addition, a varietyof shapes for the two surfaces may be employed including having bothsurfaces be flat or having non complimentary geometries. The shape ofthe surfaces 48 a, b and 54 a, b will impact the angular and sheardisplacement between the elongated members 40 a, b. The convex andconcave shape of surfaces 48 a, b and 54 a, b respectively act as arotating joint allowing for articulation as apparatus 10 experiencesangular and shear loading.

The central flexible element 50 may include one or more features onsurfaces 54 a, b that allows it to interface with the flanges 42 a, b.As shown in FIGS. 2 and 5, the central element 50 includes two ribs 52a, b on the outer surfaces 54 a, b. Each flange 42 a, b includes a slot44 a, b that corresponds to the geometry of the ribs 52 a, b such thatthe ribs are received and under certain conditions engaged therein. Asthe elongated members 40 a, b are twisted under torsional loading, theribs 52 a, b engage the slots 44 a, b acting to resist the twistingmovement. The ribs 52 a, b are oriented orthogonally to each other toallow for more consistent performance in angulation and shear. This alsoallows for implantation of the device without regard to orientation.

Over time the frictional and compressive forces resulting from thecontact between flanges 42 a, b and central element 50 will adverselyaffect the dynamic performance of element 50 due to wear anddegredation. Although the central element 50 is constructed from amaterial that resists wear, the geometry of the features on surfaces 54a, b may be varied in order to increase the durability of the centralclement 50. FIGS. 6-9 illustrate examples of geometries that may beemployed. FIGS. 2 and 6 shows a plurality of ribs 56 a, b disposed onthe surface 54 a, b such that the amount of surfaces 54 a, b thatcontact the inner surfaces 48 a, b are minimized. FIG. 7 shows thecentral flexible element 50 with centrally located polygons 58 a, b. Inaddition, the polygons 58 a, b may have any number of sides, forexample, forming a star with a plurality of points. As shown in FIG. 8the central flexible element 50 has a plurality of polygons 58 a, blocated at the perimeter of central element 50. In this embodimentspolygons 58 a, b are shown with three sides but could be any number ofsides and need not match each other and not be arranged in any pattern.

FIG. 9 shows the central flexible element 50 with a plurality ofcylindrical protrusions 60 a, b positioned in a pattern about thesurface 54 a, b of the central flexible element 50. As with the otherembodiments, the cylindrical protrusions may be arranged in any mannerand need not follow a pattern. In all of the shown embodiments in FIGS.5-9, the type of engagement features on one side of the central flexibleelement 50 need not match the engagement features on the opposite side.For example one surface 54 a a may have a cruciform 56 a while the othersurface 54 b has multiple polygons 58 b. In addition, the surfaces 54 a,b can each have different types of features such as polygons andcylindrical protrusions thereon.

As shown in FIGS. 2 and 4, a flexible element or elements 30 a, b ismounted about the proximal end of one or both of the elongated members40 a, b. The flexible elements 30 a, b comprise a collar 34 a, b and acentral region 36 a, b each interacting with housing 20 depending uponthe movement of the elongated members 40 a, b. For example, the centralregion 36 a, b of the flexible elements 30 a, b is acted upon when theelongated members 40 a, b are moved in an axial direction. The collar 34a, b may protrude slightly beyond the margin of the housing 20 and isacted upon when the elongated members 40 a, b are moved in an angular orradial direction. As with the central flexible element 50 one or both ofthe flexible elements 30 a, b may be constructed form a Newtonianmaterial whereby transverse strain in response to axial strain isdescribed by Poisson's ratio.

As shown in FIG. 10A an inner surface 32 a, b of the central region 36a, b of the collar 34 a, b contacts an outer surface 46 a, b of theflange 42 a, b when the elongated members 40 a, b are in a neutralposition, for example, when the spine is at rest. Alternatively, a spacemay exist between the surfaces 32 a, b and 46 a, b, not shown in thedrawings, to allow for greater unrestricted axial movement of elongatedmembers 40 a, b. FIG. 10B illustrates the elongated members 40 a, bsubjected to an axial force causing the members 40 a, b to move distallyin the direction of arrow 61 such as would be experience when the spineis flexed. The outer surfaces of flanges 46 a, b pushes on the flexiblemembers 30 a, b at contact surfaces 32 a, b. This in turn causesflexible members 30 a, b to come into contact with the housing 20 anddeform into interstitial space 28 and/or push the collar 34 a, b throughan opening in the ends 21 a, b of the housing 20. The expansion of theflexible members 30 a, b into space 28 and through the ends 21 a, b ofhousing 20 causes an exponential increase in resistance to axialloading. This allows for the restricted and stabilized movement ofadjacent vertebral bodies.

A variety of shapes and sizes can be employed for the outer surfaces 46a, b of the flanges 42 a, b and the corresponding contacting surface 32a, b of the flexible elements 30 a, b. The shape of these surfaces maybe varied in order to create a desired dynamic response. Providing aconcave shape on surfaces 32 a, b may lead to a more rapid deformationof the flexible elements 30 a, b and, consequentially more rapidstiffening to limit the range of motion for the elongated members 40 a,b. Alternatively, a convex shape may be utilized whereby the flanges 42a, b have a thinner profile allowing for the flexible elements 30 a, bto be larger. This may provide for a greater range of motion to theelongated members 40 a, b.

As shown in FIGS. 1-4 the housing 20 may be generally cylindrical andhas openings at each end 21 a, b allowing elongated members 40 a, b topass there through. In one embodiment of the invention, the housing 20is constructed from a generally rigid material that will not deformunder the physiological loading encountered within the spine. Thehousing 20 may be formed from a first 20 a and a second 20 b casingwherein each of the casings 20 a, 20 b have an opening at ends 21 a, b.As shown in FIG. 4, casing 20 a includes a locking feature 22 thatcorresponds to a locking feature 24 on casing 20 b so as to form a snaplock. Casing 20 b also includes a plurality of expansion slots 26 thataid in assembly of the apparatus as will be described below.

As shown in FIG. 2, the apparatus 10 is assembled by aligning theproximal ends of elongated members 40 a, b with the central flexibleelement 50 such that the protrusions 52 a, b correspond to slots 44 a,b. The flexible elements 30 a, b, which have an opening in the centercorresponding to the cross sectional geometry of elongated members 40 a,b, are then inserted over the distal ends of elongated members 40 a, band slid into place about the proximal end thereof or in proximity toflanges 42 a, b. Thereafter, the distal ends of elongated members 40 a,b are placed through the openings in the ends 21 a, b of casings 20 a,b. The openings in the ends 21 a, b are larger than the cross sectionalgeometry of the elongated members 40 a, b creating a space between theelongated members 40 a, b and the housing 20. Casings 20 a, b are placedtogether and inward pressure applied thereon such that locking features22 slides under locking feature 24 which moves in a radial direction asfacilitated by expansion slots 26 locking casings 20 a, b together. Thecasings can also be assembled by welding, bolting, threading, screwing,adhesive bonding, magnetic coupling, clamping, or twist locking. Oncecasing 20 is assembled, the proximal ends of members 40 a, b, thecentral element 50, the flexible elements 30 a, b and the flanges 42 a,bare contained therein. The collars 34 a, b of flexible elements 30 a, b,however, are located in the spaces between the housing 20 and members 40a, b and may slightly protrude through the space between the elongatedmembers 40 a, b and the openings at the ends 21 a, b of the housing 20.

FIG. 12 illustrates the apparatus 10 implanted in the spine betweenadjacent vertebral bodies 8 a, b in order to provide stability to thejoint existing between vertebrae 8 a, b. Typically, stabilizationdevices such as apparatus 10 are mounted to the vertebral bodies 8 a, bvia an anchoring device 3. Once mounted to the spine 2 the apparatus 10serves to stabilize adjacent vertebral bodies while also allowing formotion.

As shown in FIGS. 10A-B, when the spine 2 is moved in the direction ofarrows 11 the elongated members 40 a, b transfer force to the flexibleelements 30 a,b as described above allowing for a limited range ofmotion. As shown in FIGS. 11A-B and 14 when the spine is moved in thedirection of arrows 12 or placed in extension the elongated members 40a, b moved toward the central flexible element 50. The central flexibleelement 50 expands into spaces 70 and under certain conditions, forexample increased extension of the spine creating the dynamic responsediscussed above. As shown in FIG. 15, the spine 2 is experiencinglateral or side bending, as indicated by the arrows 13. Under thelateral loading shown in FIG. 15 apparatus 10 a will transfer theloading forces to flexible members 30 a, b while the apparatus 10 b willtransfer loading to the central flexible element 50.

FIG. 16 shows the spine rotated about its axis in the direction of arrow14. Due to the apparatus 10 being mounted away from the axis of thespine the device 10 experiences shear loading. As shown in FIG. 17 theelongated members 40 a, b are moved in the direction indicated by arrows63. As the elongated members 40 a, b are moved, surfaces 48 a, b and 54a, b shift relative to each other such that flange 42 a, b is free tointeract with additional flexible elements 30 a, b. As the flanges 42 a,b contact additional flexible elements 30 a, b, they impinge upon thehousing 20. Thereafter, the additional flexible elements 30 a, b resistmovement in the manner described above with reference to the distalmovement of the elongated members 40 a, b. This dynamic responsemaintains the elongated members 40 a, b roughly parallel to each otheras the central flexible element 50 and the housing 20 rotate.

Apparatus 10 has been described above primarily with reference tounidirectional loading conditions. As shown in FIG. 18, however, theapparatus 10 can handle loading in multiple directions simultaneously.For example, as shown in FIG. 13 the elongated members 40 a, b are movedboth distally and obliquely from each other causing the additionalflexible elements 30 a, b interact with the flanges 42 a, b and thehousing 20 in a manner as described above. The space between the housing20 and the elongated members 40 a, b into which collars 34 a, b areplaced allows elongated members 40 a, b a range of angular motion thatis restricted in the direction of movement by the additional flexibleelements 30 a, b whereby collars 34 a, b act as a buffer between thehousing 20 and the elongated members 40 a, b.

Although the present invention has been described above with respect toparticular preferred embodiments, it will be apparent to those skilledin the art that numerous modifications and variations can be made tothese designs without departing from the spirit or essential attributesof the present invention. Accordingly, reference should be made to theappended claims, rather than to the foregoing specification, asindicating the scope of the invention. The descriptions provided are forillustrative purposes and are not intended to limit the invention norare they intended in any way to restrict the scope, field of use orconstitute any manifest words of exclusion.

What is claimed:
 1. An apparatus comprising: at least two elongatedmembers having a distal and proximal end, the proximal end comprising aflange, wherein the flange comprises a proximal surface and a distalsurface; a first flexible element disposed between the proximal ends ofthe at least two elongated members, wherein the first flexible memberhas a first and a second outward facing surface and further wherein theproximal surface of each of the flanges interacts with at least one ofthe outward facing surfaces of the first flexible element to form ananti-torsional coupling; first and second additional flexible elements,each additional flexible element having a collar at least partiallysurrounding the proximal end of a corresponding elongated member; and ahousing having a first and a second end encapsulating the proximal endsof said members such that the first flexible element and the first andsecond additional flexible elements are contained therein wherein thefirst collar protrudes through an opening in the first end of thehousing.
 2. The apparatus of claim 1 wherein the anti-torsional couplingfurther comprises at least one protrusion on each of the outer facingsurfaces of the first flexible element and at least one correspondingrecess on the proximal surface of each of the flanges that receives saidat least one protrusion.
 3. The apparatus of claim 2 wherein the atleast one protrusion on a first of the outward facing surfaces of thefirst flexible element is oriented so as to be offset from the at leastone protrusion on a second of the outward facing surfaces of the firstflexible element at a pre-determined angle.
 4. The apparatus of claim 2wherein the at least one protrusion comprises a rib that transverses theoutward facing surface.
 5. The apparatus of claim 2 wherein the at leastone protrusion comprises at least two transverse ribs that intersect. 6.The apparatus of claim 2 wherein the at least one protrusion comprisesat least one polygon.
 7. The apparatus of claim 1 wherein the proximalsurface is generally convex and contacts a surface of the first flexibleelement.
 8. The apparatus of claim 1 wherein the distal surface isgenerally concave and contacts a surface of the at least one additionalflexible element.
 9. The apparatus of claim 1 wherein the anti-torsionalcoupling further comprises at least one recess on each of the outerfacing surfaces of the first flexible element and at least onecorresponding protrusion on the inward surface of each of the flangesthat receives said at least one recess.
 10. The apparatus of claim 1wherein the housing, the first flexible element and the first and secondadditional flexible elements cooperate to define a space to permitdeformation of the first flexible element in a lateral direction.
 11. Astabilization apparatus for implantation into a spine comprising: afirst and a second elongated member each having a distal and proximalend; an enlarged region located at the proximal end of each elongatedmember, each enlarged region having a proximal and a distal face; afirst flexible element disposed between the enlarged regions of eachelongated member, wherein the first flexible member has a first and asecond outward facing surface and further wherein the proximal face ofeach of the enlarged regions interacts with at least one of the outwardfacing surfaces of the first flexible element to form an anti-torsionalcoupling; a first and second additional flexible element at leastpartially surrounding the proximal end of a corresponding elongatedmember in proximity with the enlarged regions, wherein the first andsecond additional elements further comprise a collar; and a housingencapsulating the first flexible element, the enlarged regions, and theadditional flexible elements wherein the first flexible element andadditional flexible elements interact with the enlarged regions and thehousing to permit movement of the elongated members in a directioncomplimentary to the movement of the spine, wherein the collar of thefirst additional flexible element protrudes through an opening in afirst end of the housing.